Prior art high power triodes/tetrodes are typically limited to frequencies below 300 MHz and power levels less than 50 kW. The maximum useful frequency for amplification is limited by control grid capacitance and the physical limitation of small control grid to cathode and control grid to anode spacing required for high average power operation. The maximum output power of prior art triode/pentode devices is limited by available beam power and thermal dissipation on the control grids and anode plate.
Amplification at high power or high frequency beyond the capability of gridded triodes is typically accomplished using Klystrons or Inductive Output Tube (IOT) devices. At frequencies below 800 GHz, however, klystrons are extremely large and require magnetic focusing. IOTs, which are also magnetically focused, may have multiple electron beams, each electron beam formed from a concave thermionic cathode operating at significantly elevated temperature than a gridded triode, and requiring a graphite control grid because of the elevated operating temperature. However, it is difficult to design the magnetic field structures for such IOT devices because of the complex interactions between the solenoidal magnetic field generator and the off-axis electron beams, and the expense and fragility of fabricating graphite control grids. Multi-beam IOTs (which are only recently commercially available) are expensive, large, and difficult to fabricate, and single beam IOTs are not practical for power levels above 100 kW.
It is desired to provide a gridded triode and tetrode which do not require the magnetic field of klystrons and IOT devices, and are smaller in size than the klystron or IOT, the triode and tetrode device utilizing planar cathodes, and providing higher frequency operation and higher power operation than prior art gridded triode/pentode devices.